1. Field of the Invention
The invention relates to a high pressure discharge lamp, such as a xenon lamp, a high pressure mercury lamp, or the like. The xenon lamp is used, for example, in a projection apparatus or the like using DLP (digital light processing) technology as a light source. The high pressure mercury lamp is used, for example, as a light source in a semiconductor exposure device of a liquid crystal exposure device, a device for exposure of a printed board or the like.
2. Description of Related Art
Conventionally, a lamp with the arrangement which is shown by way of example in FIG. 3 is known as a high pressure discharge lamp. This high pressure discharge lamp 10 is made of a silica glass bulb which has a light emitting part 11 and hermetically sealing parts 12. Furthermore, the high pressure discharge lamp 10 consists of a cathode 13 and an anode 14 which are located within the light emitting part 11 opposite one another.
The tungsten upholding part 131 of the electrode supports the cathode 13 and the tungsten upholding part 141 of the electrode supports the anode 14. The upholding parts 131, 141 of the electrodes are each inserted into and held in a holding cylinder 16 which is fixed within the hermetically sealing part 12, which is cylindrical, made of silica glass and within which there is a through opening which runs in the axial direction. The upholding parts 131, 141 of the electrode are sealed by means of a graded glass 15 in the hermetically sealing part 12, extend from the outer end of the bulb to the outside, projecting over it, and also act as outer lead pins which feed power to the cathode 13 and the anode 14.
In a high pressure discharge lamp 10 with the above described arrangement the cathode 13, as shown in FIG. 4, has a cylindrical body part 132 and a conical part 134 which has the shape of a truncated cone and which is located on one end of this body part 132 integrally with it. The diameter along the axis L of this body part 132 forward (to the left in the drawing) gradually decreases and on its tip a round flat tip surface 133 is formed. This conical part 134 is made of thoriated tungsten in which tungsten, as a metal with a high melting point, has been doped with an electron emissive material of thorium dioxide (ThO2). The anode 14 is made of pure tungsten.
In this cathode 13, in its conical part 134, thorium which is produced by reduction of the doped-in thorium dioxide by the tungsten acts as the emitter; this leads to stable emission of electrons. As a result the formation of a discharge arc is simplified.
However, for the reduction of thorium dioxide by tungsten, a very high temperature of at least 2500° C. is necessary. In a normal state of use, in the area outside the tip-side area 136 which is adjacent to the discharge arc in the conical part 134—i.e., in the area 135 on the base side—it is difficult to obtain such a high temperature state. As a result, in this area 135, on the base side, the reduction reaction of thorium dioxide by tungsten does not progress. Therefore, thorium which acts as an emitter cannot be efficiently produced.
On the surface of the conical part 134, the ratio of the reduction to thorium to the total amount of doped thorium dioxide—i.e., the reduction ratio of the thorium dioxide—is low, as was described above. Therefore, thorium dioxide cannot be used with high efficiency. As a result of the fact that the absolute amount of thorium which in fact contributes to the formation of the discharge arc decreases, therefore, there is the disadvantage that in the high pressure discharge lamp 10 a long enough operating service life cannot be obtained.
In order to eliminate the above described disadvantage, it has been proposed that, in the conical part 134, the surface of the area 135 on the base side be subjected to carbonization and that, in this area 135 on the base side, surface layers of tungsten carbide (W2C), be formed (see, for example, Japanese patent disclosure document 2000-21349).
This arrangement makes it possible to reduce the thorium dioxide in a relatively low temperature range of at least roughly 1800° C.; in a normal state of use, this also leads to accomplishment of a temperature which is necessary for reduction of thorium dioxide in a wide range on the surface of the conical part 134 of the cathode 13. In this way, the degree of reduction of the thorium dioxide is increased and a large amount of thorium does in fact contribute to formation of the discharge arc. Therefore, it becomes possible to prolong the service life of the high pressure discharge lamp 10.
However, in the normal state of use, in the conical area 134, on the surface of which the tungsten carbide layers have formed, in the area 135 on the base side, there is an area in which a high enough temperature is not reached. Therefore, a high enough degree of reduction of the thorium dioxide cannot be implemented. Thus, it was found that there is the disadvantage that in this high pressure discharge lamp 10 the expected service life cannot be achieved.